NCERT Solutions for Class 9th Science Chapter 10 .6.4 Human perception of sound — In-text Questions
Book page 19910 Updated on2026-09-08
Q1.
Can humans hear all sounds?
Answer
No. Humans can hear only a limited band of frequencies — the audible range, from about 20 Hz to 20 000 Hz (20 kHz).
The human audible range is only the middle strip. Sound exists on both sides of it, but our ears do not respond to it.
Below 20 Hz the waves are called infrasonic waves. Elephants can detect them, and they are used to detect earthquakes, volcanic eruptions and severe storms.
Above 20 kHz they are called ultrasonic waves. Dogs, cats, bats and dolphins can detect them, and we use them in sonar, ultrasonography and industrial flaw detection.
Why there is a limit at all: hearing is a mechanical process. The eardrum and the tiny bones behind it must be able to follow the incoming density changes, and the hair cells in the cochlea must convert them into electrical signals. Both fail outside a certain band of frequencies. This is a limit of the detector, not of the sound — the wave is perfectly real either way.
Did you know? The upper limit falls with age, and it also falls with damage from loud sound. A teenager may hear up to 20 kHz while an adult may stop at 14 – 16 kHz, which is why the range is quoted as varying 'from person to person'.